THE FELDSPARS : THEIR COMPOSITION. 39 



by the molecular union of albite and anorthite is an established law, and 

 not a mere hypothesis. He states that the supposition that the difference 

 of composition in the plagioclastic feldspars is due to the successive addi- 

 tion of silica molecules, takes no account of the replacement of lime and 

 soda, which is so intimately associated with the variation in the amount of 

 silica. Andesite, he holds, is distinct from oligoclase. 



In 1876,* Descloizeaux described a new potash feldspar (microcline) 

 which is triclinic, although chemically the same as orthoclase. This dis- 

 covery only added to the difficulties and confusion in the feldspar question. 

 Mallard and Michel Levy, however, taught later that orthoclase and micro- 

 cline were the same, but that the cross-twinning had become so fine that it 

 was no longer visible in polarized light, i. e., the laminte were excessively 

 thin — so much so as to cause the feldspar to appear optically homo- 

 geneous.! 



Extended observations were later made by Max Schuster on the optical 

 characters of the feldspars. He claims that these characters show a gradual 

 change or transition between anorthite and albite, />«;•/ passu with the varia- 

 tion in chemical composition ; that is, each definite proportion in the mix- 

 ture of anorthite and albite gives a variety whose optical properties 

 approach one or the other of these feldspars, according to the predomi- 

 nance of either. From the optical characters of any feldspar crystal, 

 there could be inferred its chemical composition, and the reverse. He 

 claims that Tschermak's law is sustained by these observations, and such 

 seems to be the prevalent opinion, t The.se observations of Schuster are 

 in accordance with those of Sennamont on Kochelle salts. § They not im- 

 properly may lead to very different views of mineral species from those 

 commonly held. 



From the above, it seems clear that the feldspars are either species 

 with such indefinite boundaries that they (the feldspars) cannot be defined 

 with any accuracy, or else they form a continuous series from anorthite 

 to orthoclase. In either case it is improper to found definite groups and 

 specific divisions of rocks on a variable and indefinite group of minerals, 

 concerning whose nature the chemical mineralogists are not agreed. 



* Comptes Rcndus, 187fi, Ixxxii. 8S5-S91 ; Aim. Chimie Physique, 1876 (.">). ix. 433-499. 

 t Bull. Mill. Soc. France, 1879, pp. 135-139 ; Neues Jalir. Miu., 1880, L pp. 174, 175 ; Zeit. Krysi, 

 ISSO, iv. 632,633. 



X Sitz. Wien. Akad,, 1879, Ixxx. (1), 192-200; iliii. MittU., 1880 (2), iii. 117-284. 

 § Ann. Chimie, Physique, 1S51 (3), xxxiii. 429-437. 



